In order to do this, you will have to use javaScript, but Struts can certainly do some of the work. Here's one way to do it:
in your Action class:
errors.add("lastName", new ActionMessage("error.lastname");
in your jsp:
<head>
<script>
function init() {
<logic:messagesPresent property="lastName">
document.getElementById("lastNameLabel").style.color = "yellow";
</logic:messagesPresent>
}
</script>
</head>
<body>
<html:form action="myaction" on load="init()">
<div id="lastNameLabel">Last Name:</div>
<html:text property="lastName"/>
...
</html:form>
</body>
